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Other Terms: Brachial plexus, Plexus brachialis, Plexus brachial
The brachial nerve plexus is formed by the anterior divisions of the fifth, sixth, seventh, and eighth cervical nerves and dorsal nerve. Above, the plexus is reinforced by a small branch of communication which passes from the fourth to the fifth cervical nerve. Below, a similar connecting branch often passes upward from the second to the first dorsal nerve. In the neck, it consists of three trunks: an upper, middle and a lower. In the axilla, of three cords, it consists of a lateral, a medial and a posterior. The three trunks, seen in deep dissection of the side of the neck, are as follows: The upper trunk is formed by the anterior primary divisions of the fifth and sixth cervical nerves, the middle trunk by the anterior primary division of the seventh cervical, and the lower trunk by the anterior primary divisions of the eighth cervical and first dorsal nerves. These trunks lie in relation with the second and third portions of the subclavian artery. The upper and middle trunks lie above the artery, and the lower one partly behind it. These three trunks, each of which divides into an anterior and a posterior branch, enter the axilla by way of its apex, lying above and to the lateral (acromial) side of the first portion of the axillary artery. The anterior branches of the upper and middle trunks unite to form the lateral cord of the plexus. This cord lies on the lateral side of the second portion of the axillary artery. The anterior branch of the lower trunk constitutes the medial cord of the plexus, which lies on the medial side of the artery. The posterior branches of all three trunks unite to form the posterior cord of the plexus, which lies posterior to the second portion of the axillary artery. Sometimes the posterior cord is formed simply by the union of the posterior branches of the upper and middle trunks. The posterior branch of the lower trunk, small in comparison with the others, unites with the radial branch of the posterior cord. The first two stages in the formation of the plexus take place in the neck. The third is posterior to the clavicle and the last is in the axilla. The branches given off from the brachial plexus below the clavicle are: the lateral and medial pectoral nerves, the upper and lower subscapular nerves, the thoracodorsal nerve, the axillary nerve, the musculocutaneous, the median, the ulnar, the medial antebrachial cutaneous nerve, the medial brachial cutaneous, and the radial. Of these branches the lateral pectoral nerve, the lateral head of the median, and the musculocutaneous arise from the lateral cord. The subscapular nerves, the thoracodorsal nerve, the axillary nerve, and the radial nerve arise from the posterior cord. The head of the median nerve, the ulnar nerve, the medial antebrachial cutaneous nerve, the medial brachial cutaneous nerve, and the medial pectoral nerve arise from the medial cord.